Adam Clarke on Proverbs 11:17

The merciful man doeth good to his own soul - Every gracious disposition is increased while a man is exercised in showing mercy. No man can show an act of disinterested mercy without benefiting his own soul, by improving his moral feeling.

But he that is cruel troubleth his own flesh - We seldom see a peevish, fretful, vindictive man either in good health, or good plight of body. I have often heard it observed of such, "He frets his flesh off his bones."